Using FFT in MATLAB on multiple matrices of data to find differences within responses

Question

I have 4 matrices of data F1,F2,O1,O2. All are neural signals collected at 1ms for a second. F1 and O1 were collected at the same time as with F2 and O2. I need to find how the data collected differes between the 2 trials and also compare the components of each trial (F1 and O1) to each other to notice and differences in respones. I'm new to MATLAB but I believe I need to use the fft function. I'm just not sure where to start, any help would be greatly appeciated.

Answer 1

If you're trying to compare the frequency spectrums of two separate acquisitions, then fft is the tool you want to use.

Mathworks has pretty good documentation on how to use the fft function, and you can probably cut and paste your data right into the example code they provide.

If you want to plot the data on the same axes, you can use the hold on command, plot different line colors (Ex: plot(x,y,'r') will plot a red line, 'b' a blue one, etc. - see lineseries properties) and include a legend to label the plots.

Answer 2

Hi,

Based on your sampling rate (1000 times per second), you will only be able to analyze signals which have frequency of up to 500 hz. Any neural signal components which have higher components will appear as signals of lower components (unless your device has highpass filter etc..)

The command for fft from Matlab Help is: Y = fft(signal, n)

Signal is either F1 or O1 or F2 or O2 and should be a vector 1000 long. n determines how many samples your FFT have. This is essentially how finely you will split up the frequency values between 0 hz and 1000 hz (your sampling rate). For example, if you choose n =256, your Y will be a 256 long vector with a measure corresponding to the frequencies (0*1000/256 hz, 1*1000/256 hz, ... 255*1000/256 hz).

Y will be a vector of complex values. Often, you want to see the strength or power of the signal. You can use 'abs()' to find the magnitude. myPSD = abs(Y). Next because your signals are real signals, their fft's are symmetric about half the sampling rate (500hz). Thus, you should only look at the first half. Here is a code snippet to look at the first half.

Y = fft(signal, n); % choose your n

myPSD = abs(Y);

myHalfPSD = myPSD(1:ceil(n/2))

myFreqValues = [0:1000/n:500-1000/n] % both myHalfPSD and myFreqValues should be n/2 long

plot(myFreqValues, myHalfPSD)

Typically, PSD is displayed on a log scale or even decibal. So you might add a line.

Y = fft(signal, n); % choose your n

myPSD = abs(Y);

myHalfPSD = myPSD(1:ceil(n/2))

myHalfDBPSD = 20*log(myHalfPSD)

myFreqValues = [0:1000/n:500-1000/n] % both myHalfPSD and myFreqValues should be n/2 long

plot(myFreqValues, myHalfDBPSD)

If you want to plot all 4 graphs at once you might want to use something like subplot(4,1,1), subplot(4,1,2) etc..

Hope that helps,

Chuan